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JfHirmfi ifj ShiKjhh Resenrdj, Vol 20, Nn. ]. 143-147, 2(1(11. 

(;R0WTH of BL TTKR clams, SAXIDOMLS GIGANTEUS DESH WES, ON SELECTED 

BEACHES IN THE STATE OF WASHINGTON 



STUART ALAN GOONG AND KENNETH K. CHEW 

UnireKsity of Washiu^iton Sihool of Aquarit ami tishery Svn'iiccs, Box 355020, Seaitle, 
Wmhini^fofi 98195 

ABSTiL\CT The huner clam, Stixidimufs ^iii^iiaiueiis Deshiiyes, is a highly VLilued reLTeaiiunui shellfish species, but is currently of 
liitle coiiiniercial importance in Washington. Reeciuly, a small-sciile Lommercial butter clam harvest began* and interest in this species 
is expected to increase. Basic intormaiion on the biology and ecology of the clam is necessary to establi>^h a soynd manfigemerit regime 
for recreatiorril and potential commercial hardest of butttr clams, Beaches studiLd were Birch Bay State Park. Dnuble Bluffs, and 
Poibtch State Park Grnwth raie^ were deicmiined by meiisuring kngihs-at-ttge lor cljiiis ctjIleLted Irnm each beach VtJri Bertaianfty 
grtmlh curves were produced using nonlinear regreiision analysis. Growth ralo were signitkantlv different amnng the three bCLichcs. 
with the fiistesi growth occurring at Double Bluffs, ft? I lowed by Bnch Bay State Park, while ciams at Pot latch Stale Park had I he slowest 
grow til. 

KEY WORDS: Saxidotmis ^i^anieus, grou [3i 



INIKODUCIION 

The purpose of this investigation was to assess whether growth 
of butter clams {Snxidomus gigutiteus Deshayes) c^n selected 
beaches in Washington tliffercd Beaches selected for this study 
were Birch Bay State Park. Double Bluffs, an Island County rec- 
reational area on the southwestern coast of Whidbey Island. VVush- 
ington. and Potlatch State Park (Fig. I). There was no evidence to 
suggest ihiil tiroulh rates niit^bt differ uithin the Pu^ct Sound 
Basin, although dilTerent growth ratc^ have been observed for 
butter clams from beaches widiin close proximity in British Co- 
lumbia (Quaylc and Bourne 1972). 

The three beaches incltided in this study were surveyed for 
butter clams using methods similar to beach survey methods used 
by the Washington Depanmem of Fish and Wildlife fWDFW). 
Clam shells were aged using the annular method, which appears to 
be the most reliable method of age determination for butter clatns 
tGoong 1 999 1. Ai least two researchers have independently veri- 
fied deposition of annuli in the shell of the butter clam us»ing mark 
and recapture studies in Washington and British Columbia 
(Houghton 1973, Neil Bourne pers. comm.). Growth rates were 
calculated using von Bertalanffy methodology (von Bertalanffy 
1938). 

iVIATRRI VLS ANI> METHODS 

Study Areas and Ha hi! a f 

Two hundred and forty-one butler clams were collected from 
three beaches for this study from August 17-19. 1997. Clams were 
collected from Birch Bay State Park on August 17, from Double 
Bluffs on August IS, and from Potlatch Slate Paik on AuL^iist 19 

Birch Bay Stale Park is located near Belliugham at the southern 
end of the Strait of Georgia. The beach has a gradual slope, and is 
characterized as primarily a sand-rock beach, with numerous larger 
cobblestones (up to 15 cm x 15 cm) on the surface. The beach has 
good road access, w ith parking less thati 50 m from the water. This 
allows heavy recreational use* including clam harvesting, judging 
from the large ntimber of clam harvesters observed and poor clam 
densities found on the beach. Other clam species observed on the 
beach included many Manila clams {Tapes phi! ipphui mm) and a 
few native littleneck clams {Pwunhaca staminea). Green shore 
crabs iHemrf^rapms sp. ) w^ere common. Water quality measure- 



ments, as recorded by tlic Washington Department of Ecology 
(WDOE) (Newton et al. 1997), showed surface tempera lures av- 
eraged 7 'C in winter and 1 0-1 3°C in summer. Salinity was fairly 
constaiiL ranging from 24-27 ppi throughout the year. The con- 
centration of chlorophyll a was about 0.6 pig/l- in winter, and 
about 3 fJLg/L during the rest of the year. 

Double Bluffs Beach is located on the southwestern coast of 
Whidbey [skmd, in central Puget Sound Substrate and topography 
of this beach was similar to that of Birch Bay. There was poor road 
access, with a parking area located approximately 1 km from the 
southern boundary of the clam bed. This beach probably has little 
recreational harvest, judging from the few clam harvesters ob- 
served, high clam densities, and large numbers of older clams 
found there. Other species of clams observed on this beach in- 
cluded small nuTTibers of Manila clams, cockles (Clirtocardiuw 
ininallii bent nose clams {Mistrmui fHisani), and horse clams {Tre- 
Sits capax}. There were also large numbers of green shore crabs 
and acom barnacles {BiilafiiLs sp.j. One juvenile cancrid crab was 
also observed. Sea stirface temperatures averaged 8"C in winter 
and 10-12'^C in summer, a tcmpciature regime similar to that of 
Birch Bay. Salijiity averaged 29 ppt throughout the year. Chloro- 
phyll d concentrations were about 4 |j.g/L in winter, and about 
43 |xg/L in sutnmer. These water quality measurements were simi- 
lar to those found at the WDOE sampling station near Birch Bay. 
Lower salinity measurements for the station near Birch Bay may 
be due to the influence of freshwater from the Eraser River in 
southeni British Columbia (Newton et al. 1997). 

Potlatch State Park h at the southern end of Hood Canal near 
the southern end of the Puget Sound Basin, The topography of this 
beach was more com [ilex than ihe other two beaches. Much of the 
beach was flat near the road, then sloped gradtialiy upward toward 
the water, and then downward. However, some portions of the 
beach sloped gradually downward the entire distance to the water. 
Because of ihis, much of the flat area of the beach is flooded 
during high tide and had good clam populations, despite being 
fairly distant (50 m or more) from the water level during minus 
tides. Substrate of this beach was best characterized as mud-graveL 
and there were no cobblestt>nes on the surface. Sampling of this 
beach coincided with the tlnal day of a three-day native tribal 
Manila clam harvest, during which butter clams were also col- 
lected. Thus, clam densities were poor, and most clams collected 



143 



144 



GooNri AND Chew 



124*^ W I \'%\ U I 12J^W 



VANCOUVER 
ISLAND 



PACIFIC 

OCEAN 

h 47° N 






''^-^J 




Befl Ingham 




verett 



Seattle 



Tacoma 



Figure 1. Map of the Greater Pu^et Sound Region showing 1r)c:atinn«i of thi' liiree study sitfS. 



were in ihe 3+ age class or younger Other clam species observed 
on thib beach included a (evv bent nose clams and nujiierous Ma- 
nila ciams. Surface temperatures averaged 7^ in winter and 14- 
IS'^'C in summer. Salinily was highly variable, due to freshwater 
drainage from the Skokomish River and several small creek svs- 
lems, but ranged from 16=26 ppt. Chlorophyll a concentrations 
were also highly variable, but tended to be quite low during sum- 
mer months, averaging only abtutt i).^) ^ig/L. The two other 
beaches had steady c once jii rat ions of chlorophyll a from spring 
through autumn, with low chlorophyll a concentrations only in 
winter months. 

Samplhii; Mvtiiads 

Sampling was undertaken using survey methods sinnlar lo 
those employed bv WDKW (Campbell 19%). A reference transect 
was established on each beach which ran perpendicular to the 
shoreline. The end of the transect nearest the high water line was 
situated approximately 30.5 m Irom the water line at the low tide 
mark. A 30.5 m length of rope was laid along the transect down to 
the water line. The rope was marked with a permanent i linker 
every 1 .5 m so it could be used to measure distances on the beach. 
Quadrats of 0.25 m^ in area were established every 4.5-10.5 m 
along the length of the transect to ensure that clams were collected 
from different tidal heights Positions of the quadrats occasionally 
had to be adjusted in order to accoiml for beach topography. Sub- 
sequent transects were established 9-15 m from each other and the 
reference transect, and were also perpendicular to the shoreline. 



Quadrats were established Lilong these transects in a similar man- 
ner and excavated to a depth of 0.5 m. 

Quadrats were excavated using standard garden spades. As 
quadrats weie excavated, rocks and other large debris were re- 
moved. Large butter clams were removed and collected. The re- 
mainder of the substrate was sifted through I -cm mesh screens in 
order to collect smaller clams. 

Ariatysis of Growth 

Data were analyzed using the von Bertalanffy growth model 
{von Beiialanffy 1938): 

1 ( t) = U - L^e-^ ^' ' "^K 

1 {t): length at age t 

e: base of the natural logarithm 

tc,: dme when length is theoretically zero 

k; von Bertalanffy growth cim statu 

L^: asymptotic maximum length of the clam. 

Model paiamcters were estimated using nonlinear regression 
analysis, as suggested by Gallucci and Quinn ( IQ79), in lieu of the 
Ford-Waiford method. Gallucci and Quinn ( 1979) noted that use of 
nonlinear regression facilitates quantitative comparisons among 
the parameters of the equation. Connnonly, comparisons are made 
on the growth constant {k} only. Since it is standard practice to 
report differences in k as representing differences in growth, this 
convention was followed in the present study. An F test was uti- 
lized in order to make statistical comparisons among parameters 



Growth of Saxumjmus gigameus i^ Washington 



145 



cNeterct al. 1996). Staiisticiil iiiuit>^L^ wcvc performed with SPSS 
bAAS lor the Power Macintosh. 

RESULTS 

Vtni RtrUiiatjffy Gnmffi Anafysh 

Von Bcrt;ilantfy growth models resLiUiiii! from these piaranv 
elers are plotted in figure 2, Portions ol these curves correspond- 
ing to the first 9.5 years of growth are redrawn in Figure 3 Lo 
exhibit greuter detail. 

Figure 3 clearly sht)ws that growth is different anitmg beaches. 
Fastest growth occurred at Double Blutfs, followed by Birch Bay. 
while slowest growth was observed at Potlatch. However, although 
the rate of growth was slower at Pot latch, it did not appear to 
decrease over time as quickly as it did at the i>ther beaches. This, 
is probably due to an inadequate number of older ^pecitncns in the 
sample from Poilatch. Most clams collected from Potlatch were in 
the 3.5-vear a*ie class or yountrer. The result is a pauciiv of growth 
data beyond the first tour years, possibly leading to a poiirly fitting 
growth model One indicator of this possibility is the L-, calculated 
for Potlatch shown in Table 1 . This is an unreasonably large value 
for maximum clani length. However, the early part of the curve fits 
the data. Thus, the model may still adequately describe growth for 
the first several years. 

The growth c<instants (k) listed in Table 1 support conclusions 
drawn from Figure 3. Fastest growth occurred at Double Bluffs, 
while slowest growth occurred at Potlatch. Differences amony the 
three beaches were signitlcanl at the « = 0.05 leveL 

DISCISSION 

DiJferi'Rces Among tin' three Benches 

Of the three beaches, Potlatch is the most diverse. Potlatch is a 
rmid-gravei beach, which is somewhat different from the preferred 
butter clam substrate <il sand, shell and gravel jQuayle and Bourne 
1972). The substrate at Potlatch is more suited to Manila clams (7. 
philippiniintm), as evidenced by the numbers of Manila clams 
observed on this beach. Dottble Bluffs, where the fastest growth 
occurred, and Birch Bay are both sand-rock or sand-rock-shell 
beaches. In addition, butter clams were most abundant at Double 
Bluffs. 

Potlatch is more influenced by freshwater drainage than the 
other two beaches. Results of the present study indicate Miat butter 



50 



160 
140 
120 

too 



'Potlatch 



.--'"Bircti Bay 
— — DoubJe BluNs 








10 15 

Age (years) 



20 



zs 



30 



Figure 2. Von Bertalan1T> growth curvt*s for butirr clams from Pol- 
liiU'h Stall' Park. Birth Bav StaU- Park, and Double Blutis Btach. 



80 + 
60 4 



■ Potlatch 
--*— -Birch Bay 
Doubts Bluffs 




0.5 3.5 6.5 

Age (years) 

Fijjiire 5- Vtm Hirtiibnfly curves fr>r huttt-r Hams from Potlatch State 
Park, Birth ikn Stiitc l*ark. and nimble Klulf^ Hciicb, Waslijiigtrvn, 
plolti^d to ^.5 >ears. 

clam growth may be positively correlated with salinity. Although 
no other butter clam studies support tliis suggestion, studies on 
oysters suggest that oyster growth may be intluenced by differ- 
ences in salinity (Tore et al. 1995, Maltonee 1989). Roben ei al. 
( I ^)4? I conducted a study of Manila clam growth al three sites, one 
oj which was oceanic and the others were estuarine. Clam growth 
was fastest at the oceanic station, and they speculated that differ- 
ences in growth were due in pari to differences in salinity. Like 
butter clams, Manila clams are vencrid clams. Bardach et al. 
j 1972) reported that the optimum salinity range for Manila clams 
was abotil 2^32 ppi. which is the normal salinity range for Puget 
Sound, This salinity range may alst> be ofnimal for butter clams. 
Average salinities recorded at Potlatch are normally below this 
range, ranging from 16-26 ppt (Newton et aL 1997). 

As noted in Study Areas and Habitat, surface water tempera- 
tures at Potlatch tended to have greater fluctuations than tempera- 
tLires at the oiiier beaches. This may be important if butter clams 
are better adapted to more stable growing areas. Further, water 
temperatures in summer at Potlatch can be particularly high rela- 
tive to those at the other beaches Irregularity in year round water 
temperatures and excessive summer temperatures could cause a 
sufficient amount of stress to have a negative impact on growth. 
Temperature has been shown to be an important detemiinant of 
bivalve growth in several species, includmg another venerid clam, 
the northern quahog {Mercenaria ynercenaha). scallops, and oys- 
ters (Crockett 19HH. Claereboudt 1994. Torn et al. 1995). Mann 
and Glomb (1978) and Mann ( 1979) showed a negative correlation 
between temperature and growth in Manila clams, with fastest 
growth occurring at 1 2^. Grow th was also correlated with water 

TABLE 1. 

Paraiuttcrs oi ihi ion Bcrlidanfh {innUh tC|iiati(>ii fur butter dams 
from three beaches in \Vashiri;iton, calculated using all 

ynnuluii lenglhs. 



\Va*shiii^tnu. 



Bviich 


k±^E 


L^ ± SE 


t,, + SE 


Potlaich Siate 








Park 


0.064 + 0.010 


1(^5.903 + 20.243 


-0.222 + o.ms 


Birch Bav Slate 








Piirk 


I57±0,UI0 


93.870 ±3.21 8 


-0.083 ±{1051 


Double Bluffs 








BeLiLh 


0.1 72 ±0.006 


96.905+ L542 


0.fU3 + 0.032 


Ttital 


0.140 + 0.004 


I05J57± 1. 892 


-0.004 ±01)26 



146 



GOONG AND ChHW 



temperature in Btsurne (1982), bui the Lorrelaticm was posiUve. 
Fastest giowlh in iiatLiial popukuions of Manila ckims occurred in 
the Struit o I Georgia, where surface water icmperatiires are h\gh- 
esi, Lommonl> above 15 C in summer (fk>l]isier and Sandes 
1972). However, Mann's results were from laboratory experi- 
ments, and some other factor may be responsible for the observed 
differences in growth in British Cokimbia. 

In addition, food availability was more irregular at Pollatch, as 
indicated by chlorophyll a concentrations. There appears to be 
higher food availability during spring and autumn, Winters ai Pol- 
latch arc limes of low food availability. Unlike the other beaches, 
there was also low food availability at Potlatch during summer. At 
Diuible Bluffs and Birch Bay. food availability was consistent 
from spriui^ through autumn. This is supported hy the dissolved 
inorganic nitrogen levels at PotLilch, which are frecjuently unde- 
tectable from May through August in iho surface water (Newton et 
al. 1997). Dissolved inorganic nitrogen is generally considered the 
limiting nuuienl in marine ecosystems ( Valiela 1984 1. Thus, maxi- 
mum primary productivity al Potlatch may be a growth I i mil at ion 
for bivalves at Potlatch. Fo<id availability has been shown to be 
among the mosl imporiani determinants of bivalve growth, includ- 
ing growth in M. nienettiiria (Claereboudt 1994. Mai lo nee ]9S9, 
Crockett \9^^)^ Foe and Knight U^^S) found that Asiatic clams 
( Corhia t hi fli m ? hi ea ) a re proba bl y food I i m i te d i n the S ac ra men to - 
San Joaquin Delia, even with chlorcjphyll a concentrations as high 
as 62.5 tig/L. which is an order of magnitude higher than concen- 
trations found in Puget Sound. 

Finally, there is also a gradient of human activity al these ihree 
beaches. Pollatch may be a site of intense harvesting, il ntil for 
great numbers of butter clams, then certainly for Manila clams. 
Also as noted earlier, there was a commercial harvesi by Native 
American harvesters. Birch Bay is not subjeci lo commercial har- 
vesu but is heavily used for recreational purposes, since the road- 
way allows direct road access to the beach Double Bluffs is also 
not available to commercial harvesting and probably has little 
recreational use of the butter ckim resource due lo distance of the 
clam bed from the road access. One effect of heavy use may be 
substrate compaction. Studies conducted in Washington demon- 
strated that substrate compaciurn may be an importLint cause of low 
productivity on clam beaches {Toba et aL I9^J2), Increased human 
activity in an area has been shown lo decrease biological integrity, 
resulting in decreased species diversity, and may have negative 
impacls on growth and health in general on all tax a in a gi\en 
ecosvstem (Karr l^>8l). HoLU^hton (1973) showed a high cone I a- 
titvn between taxa richness and growing areas with high clam den- 
sities, strong growth, and high pn>ductivity. Thus, bivalve growth 
may be negatively coiTclated with a gradient of human activity. 



Indeed, tliis may be an important facior in growth on ptipular 
recreational clam beaches, although it is probably negligible on 
more remote beaches, such as Double Bluffs. 

Patch iness in age disLrihulion may have affected growth an;ily- 
sis for Potlatch Stale Park. In the von Bertalanffy analysis for 
Pollatch, the lack of strong representation of older clams likely 
contributed k> the computation of an ujuealistic L^. causing the 
Potlatch curve to eventually cross the others. If the true asymptote 
for Pollatch is closer to those of the other beaches as is suspected, 
the curve would probably not cross the others. Il should be ntUed 
thai the calculated parameicrs are still adequate to describe the 
observed growth for al least the first four years. Due to poor data 
for subsequent years, the model doe;^ not provide an adequate 
desci'iption of growth for later years. 

CONCLUSIONS 

1 Butter clams lri>m dilTerent beaches in Puget Sound. Wash- 
ington have different lates of growth. Fastest growth oc- 
curred at Double Bin It's, and slowest growth occurred al 
Poikiich. 

2. The legal si/e limit k>r bulter clams in Washington probably 
should be increased. The current size limit for bulter ckims 
is 3K ninr According to results of growth analysis, ctams in 
this si/e class would be only about 2-3 years <ild. It is 
suggested that ihe size limit be increased to about 63 mm, 
the size limit in British Columbia. Clams in this size class 
would be abtmt six years old, and would have had oppor- 
luniiies to spawn. 

3. Since rates of growth are different in diffeieni growing ar- 
eas, il is suggested thai a more localized management ap- 
proach tor Ihe resource be utilized in order to protect the 
pioductivity of the stock. A generalized management strat- 
egy will not optimally manage the resource. This would 
require growth analyses to be performed in any area that has 
any commercial or intense recreational harvests for butter 
clams. 

ACKNOWl rnCMFNTS 

The senior author wishes to express his appreciation lo his 
Supervisory Committee for their invaluable guidance. Drs. Ken- 
neth Chew. Neil Bourne, and Loveday Conquest. Thanks also to 
my Held crews for all of their hard work. Finally, special thanks to 
the (jraduale School of the University of Washington, the 
Loosanoff Foundation, and the Research, Admissions, and Schol- 
arship Committee of the School of Fisheries at the University of 
Washington for iheir generous financial support 



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The following text is generated from uncorrected OCR. 
[Begin Page: Page 143] 

Jdiimal of Slu-llfiili Reseanh. Vol. 20. No. I. I4,VI47, :()01 . 

(IROWTH OF BUTTER CLAMS, SAXIDOMVS GIGANTEUS DESHAYES, ON SELECTED 
BEACHES IN THE STATE OF WASHINGTON 

STUART ALAN GOONG AND KENNETH K. CHEW 

University of Washington School oj Aquatic and Fishery Sciences. Box 355020, Seattle, 
Washiniiton y.S'/y.f 



ABSTRACT The butter elam, Sci\icii>iiui.\ .vw Deshayes. is a highly valued recreational shellfish speeies, hut is 

cLirrently ot 

little commercial importance in Washington. Recently, a small-scale commercial butter clam harvest began, and 
interest in this species 

is expected to increase. Basic information on the biology and ecology of the clam is necessary to establish a sound 
management regime 

for recreational and potential commercial harvest of butter clams. Beaches studied were Birch Bay State Park. 
Double Bluffs, and 

Potlatch State Park. Growth rates were determined by measuring lengths-at-age for clams collected from each 
beach. Von Bertalanffy 

growth curves were produced using nonlinear regression analysis. Growth rates were significantly different among 
the three beaches, 

with the fastest growth occurring at Double Bluffs, followed by Birch Bay State Park, while clams at Potlatch Slate 
Park had the slowest 

growth. 



KEY WORDS: Sti.xiiliinnis gigaiileus. growth 



INTRODUCTION 

The purpose of this investigation was to assess whether growth 
of butter clam,s (Saxidomus giganteus Deshayes) on selected 
beaches in Washington differed. Beaches selected for this study 
were Birch Bay State Park, Double Bluffs, an Island County rec- 
reational area on the southwestern coast of Whidbey Island, Wash- 
ington, and Potlatch State Park (Fig. 1 ). There was no evidence to 
suggest that growth rates might differ within the Puget Sound 
Basin, although different growth rates have been observed for 
butter clanis from beaches within close proximity in British Co- 
lumbia (Ouayle and Bourne 1972). 

The three beaches included in this study were surveyed for 
butter clams using methods similar to beach survey methods used 
by the Washington Department of Fish and Wildlife (WDFW). 
Clam shells were aged using the annular method, which appears to 
be the most reliable method of age determination for butter clams 
(Goong 1 999). At least two researchers have independently veri- 
fied deposition of annuli in the shell of the butter clam using mark 
and recapture studies in Washington and British Columbia 
(Houghton 1973, Neil Bourne pers. comm.). Growth rates were 
calculated using von Bertalanffy methodology (von Bertalanffy 
1938). 

MATERIALS AND METHODS 

Study Areas and Habitat 



Two hundred and forty-one butter clams were collected from 
three beaches for this study from August 1 7-1 9, 1 997. Clams were 
collected from Birch Bay State Park on August 17. from Double 
Bluffs on August 1 8, and from Potlatch Slate Park on August 1 9. 

Birch Bay State Park is located near Bellingham at the southern 
end of the Strait of Georgia. The beach has a gradual slope, and is 
characterized as primarily a sand-rock beach, with numerous larger 
cobblestones (up to 13 cm x 15 cm) on the surface. The beach has 
good road access, with parking less than 50 m from the water. This 
allows heavy recreational use, including clam harvesting, judging 
from the large number of clam harvesters observed and poor clam 
densities found on the beach. Other clam species observed on the 
beach included many Manila clams (Tapes philippinarum) and a 
few native littleneck clams (Protothaca staminea). Green shore 
crabs {Hemigrapsiis sp.) were common. Water quality measure- 
ments, as recorded by the Washington Department of Ecology 
(WDOEI (Newton et al. 1997). showed surface temperatures av- 
eraged 7°C in winter and 10-13°G in summer. Salinity was fairly 
constant, ranging from 24-27 ppt throughout the year. The con- 
centration of chlorophyll a was about 0.6 |xg/L in winter, and 
about 3 p.g/L during the rest of the year. 

Double Bluffs Beach is located on the southwestern coast of 
Whidbey Island, in central Puget Sound. Substrate and topography 
of this beach was similar to that of Birch Bay. There was poor road 
access, with a parking area located approximately I km from the 



southern boundary of the clam bed. This beach probably has little 
recreational harvest, judging from the few clam harvesters ob- 
served, high clam densities, and large nulnbers of older clams 
found there. Other species of clams observed on this beach In- 
cluded small numbers of Manila clams, cockles {Chnocardlum 
llltttalll). bent nose clams (Macollul llasllla). and horse clams {Tre- 
sus cllpax). There were also large numbers of green shore crabs 
and acorn barnacles (Bakmus sp.). One juvenile cancrld crab was 
also observed. Sea surface temperatures averaged 8'C In winter 
and 10-12°C In summer, a temperature regime similar to that of 
Birch Bay. Salinity averaged 29 ppt throughout the year. Chloro- 
phyll a concentrations were about 0.4 |j.g/L In winter, and about 
4.3 p-g/L In summer. These water quality measurements were simi- 
lar to those found at the WDOE sampling station near Birch Bay. 
Lower salinity measurements for the station near Birch Bay may 
be due to the Influence of freshwater from the Eraser River In 
southern British Columbia (Newton et al. 1997). 

Potlatch State Park Is at the southern end of Hood Canal near 
the southern end of the Puget Sound Basin. The topography of this 
beach was more complex than the other two beaches. Much of the 
beach was flat near the road, then sloped gradually upward toward 
the water, and then downward. However, some portions of the 
beach sloped gradually downward the entll'e distance to the water. 
Because of this, much of the flat area of the beach Is flooded 
during high tide and had good clam populations, despite being 
fairly distant (50 m or more) from the water level during minus 
tides. Substrate of this beach was best characterized as mud-gravel, 
and there were no cobblestones on the surface. Sampling of this 



beach coincided witln tine final day of a tliree-day native tribal 
Manila clam harvest, during which butter clams were also col- 
lected. Thus, clam densities were poor, and most clams collected 



143 



[Begin Page: Page 144] 

144 

GooNG AND Chew 

124°WI"'^"'^'^xU I 12'^W 

VANCOUVER s?BJErt|Bay' 

ISLAND V>c\i:'jSLSi 

Bellingham 

Everett 

Seattle 

PACIFIC 
OCEAN 



47°N 



Tacoma 

Figure 1 . Map of the Greater Puget Sound Region sliowing iocations of tine tlnree study sites. 

were in tlie 3+ age ciass or younger. Otiier ciam species observed 
on tiiis beacli inciuded a few bent nose ciams and numerous Ma- 
niia ciams. Surface temperatures averaged 7°C in winter and 14 — 
18°C in summer. Saiinity was liigliiy variabie, due to fresliwater 
drainage from tlie Skokomisli River and severai smaii creek sys- 
tems, but ranged from 16-26 ppt. Clnioropliyii a concentrations 
were aiso liigliiy variable, but tended to be quite low during sum- 
mer months, averaging only about 0.9 p,g/L. The two other 
beaches had steady concentrations of chlorophyll u from spring 
through autumn, with low chlorophyll (( concenti-ations only in 
w inter months. 

Sampling Methods 

Sampling was undertaken using survey methods similar to 
those employed by WDFW (Campbell 1 996). A reference transect 
was established on each beach which ran perpendicular to the 
shoreline. The end of the transect nearest the high water line was 
situated approximately 30.5 m from the water line at the low tide 
mark. A 30.5 m length of rope was laid along the transect down to 
the water line. The rope was marked with a permanent marker 
every 1 .5 m so it could be used to measure distances on the beach. 



Quadrats of 0.25 m" in area were established every 4.5-10.5 m 
aiong tine iengtii of tine transect to ensure tiiat ciams were coiiected 
from different tidai lieiglits. Positions of tine quadrats occasionally 
had to be adjusted in order to account for beach topography. Sub- 
sequent transects were established 9-15 m from each other and the 
reference transect, and were also perpendicular to the shoreline. 

Quadrats were established along these transects in a similar man- 
ner and excavated to a depth of 0.5 m. 

Quadrats were excavated using standard garden spades. As 
quadrats were excavated, rocks and other large debris were re- 
moved. Large butter clams were removed and collected. The re- 
mainder of the substrate was sifted through 1 -cm mesh screens in 
order to collect smaller clams. 

Analysis of Growth 

Data were analyzed using the von Bertalanffy growth model 
(von Bertalanffy 1938): 

1 (t) = L,_ - L.'^e - "". 

1 (t); 

e: 



length at age t 



base of the natural logarithm 

time when length is theoretically zero 

von Bertalanffy growth constant 

asymptotic maximum length of the clam. 

Model parameters were estimated using nonlinear regression 
analysis, as suggested by Gallucci and Quinn ( 1 979). in lieu of the 
Ford-Walford method. Gallucci and Quinn ( 1979) noted that use of 
nonlinear regression facilitates quantitative comparisons among 
the parameters of the equation. Commonly, comparisons are made 
on the growth constant (k) only. Since it is standard practice to 
report differences in k as representing differences in growth, this 
convention was followed in the present study. An F test was uti- 
lized in order to make statistical comparisons among parameters 



[Begin Page: Page 145] 



Growth of S.widomus gig.anteus in Washington 



145 



(Neterel al. 1996). Statistical analyses were performed vMtIi SPSS 
6.1 . IS tor the Power Macintosh. 



RESULTS 

Von Bertalanffy Growth Analysis 

Von Bertalanffy growth models resulting from these param- 
eters are plotted in Figure 2. Portions of these curves correspond- 
ing to the first 9.5 years of growth are redrawn in Figure 3 to 
exhibit greater detail. 

Figure 3 clearly shows that growth is different among beaches. 
Fastest growth occurred at Double Bluffs, followed by Birch Bay. 
while slowest growth was observed at Potlatch. However, although 
the rate of growth was slower at Potlatch. it did not appear to 
decrease over time as quickly as it did at the other beaches. This 
is probably due to an inadequate number of older specimens in the 
sample from Potlatch. Most clams collected from Potlatch were in 
the 3.5-year age class or younger. The result is a paucity of growth 
data beyond the first four years, possibly leading to a poorly fitting 
growth model. One indicator of this possibility is the L, calculated 
for Potlatch shown in Table 1 . This is an unreasonably large value 
for maximum clam length. However, the early pun of the curve fits 
the data. Thus, the model may still adequately describe growth for 
the first several years. 

The growth constants (k) listed in Table 1 support conclusions 
drawn from Figure 3. Fastest growth occurred at Double Bluffs, 
while slowest growth occurred at Potlatch. Differences among the 



three beaches were significant at the a = 0.05 ievei. 

DISCUSSION 

Differences Among the Three Beaches 

Of the three beaches. Potiatch is the most diverse. Potiatch is a 
mud-gravei beach, which is somewhat different from the preferred 
butter ciam substrate of sand, sheii, and gravei (Ouayie and Bourne 
1972). The substrate at Potiatch is more suited to Manila clams (7". 
philippinanim). as evidenced by the numbers of Manila clams 
observed on this beach. Double Bluffs, where the fastest growth 
occurred, and Birch Bay are both sand-rock or sand-rock-shell 
beaches. In addition, butter clams were most abundant at Double 
Bluffs. 

Potiatch is more influenced by freshwater drainage than the 
other two beaches. Results of the present study indicate that butter 

15 

Age (years) 

Figure 2. Von Bertalanffy growth curves for butter tianis from Pot- 
iatch Stale Park, Birch Bay State Park, and Double Bluffs Beach. 
Washington. 

0.5 3.5 6.5 9.5 

Age (years) 



Figure ^. \ on Bertalanffy tur\es for butler clams from Potlatch State 
Park, Birch Bay State Park, and Double Bluffs Beach, Washington, 
plotted to 9.5 years. 

clam growth may be positively correlated with salinity. Although 
no other butter clam studies support this suggestion, studies on 
oysters suggest that oyster growth may be influenced by differ- 
ences in salinity (Toro et al. 1995. Mallonee 1989). Robert et al. 
( 1993) conducted a study of Manila clam growth at three sites, one 
of which was oceanic and the others were estuarine. Clam growth 
was fastest at the oceanic station, and they speculated that differ- 
ences in growth were due in part to differences in salinity. Like 
butter clams. Manila clams are venerid clams. Bardach et al. 
( 1 972) reported that the optimum salinity range for Manila clains 
was about 24-32 ppt. which is the normal salinity range for Puget 
Sound. This salinity range may also be optimal for butter clams. 
Average salinities recorded at Potlatch are normally below this 
range, ranging from 16-26 ppt (Newton et al. 1997). 

As noted in Study Areas and Habitat, surface water tempera- 
tures at Potlatch tended to have greater fluctuations than tempera- 
tures at the other beaches. This may be important if butter clams 
are better adapted to more stable growing areas. Further, water 
temperatures in summer at Potlatch can be particularly high rela- 
tive to those at the other beaches. Irregularity in year round water 
temperatures and excessive summer temperatures could cause a 
sufficient amount of stress to have a negative impact on growth. 



Temperature has been shown to be an important determinant of 
bivaive growth in severai species, inciuding another venerid ciam, 
the northern quahog (Mercenunu mercenaria), scaiiops, and oys- 
ters (Crockett 1988. Ciaereboudt 1994, Toro et ai. 1995). Mann 
and Giomb (1978) and Mann (1979) showed a negative cortelalion 
between temperature and growth in Manila clams, with fastest 
growth occurring at 12°C. Growth was also correlated with water 

T.\BLE 1. 

Parameters of the von Bertalanff> growth equation for butter clams 

from three beaches in W ashlngton. calculated using all 

annulus lengths. 

Beach 



:SE 



L, ±SE 



t„ + SE 



Potlatch State 



Park 0.064 + 0.010 ^.^^.903 ± 20.243 -0.222 + 0.048 



Birch Bav State 



Park 0.1 57 ±0-01 93.S70+ 3.21 8 -0.083 + 0.0. -il 



Double Bluffs 



Beach 0. 1 72 -h 0.006 96.905 + 1 .542 0.043 ± 0.032 



Total 0.140±0.0()4 105.157 + 1.892 -0.004 + 0.026 



[Begin Page: Page 146] 



146 



GooNG AND Chew 

teinpeiature in Boiinic (1982). but the correUition was positive. 
Fastest growth in natural populations of Manila clams occurred in 
the Strait of Georgia, where surface water temperatures are high- 
est, commonly above 15"C in summer (Hollister and Sandes 
1972). However. Mann's results were from laboratory experi- 
ments, and some other factor may be responsible for the observed 
differences in growth in British Columbia. 

In addition, food availability was more irregular at Potlatch. as 
indicated by chlorophyll a concentrations. There appears to be 
higher food availability during spring and autumn. Winters at Pot- 
latch are times of low food availability. Unlike the other beaches. 



there was also low food availability at Potlatch during summer. At 
Double Bluffs and Birch Bay. food availability was consistent 
from spring through autumn. This is supponed by the dissolved 
inorganic nitrogen levels at Potlatch. which are frequently unde- 
tectable from May through August in the surface water (Newton et 
al. 1997). Dissolved inorganic nitrogen is generally considered the 
limiting nutrient in marine ecosystems (Valiela 1984). Thus, maxi- 
mum primary productivity at Potlatch may be a growth limitation 
for bivalves at Potlatch. Food availability has been shown to be 
among the most important determinants of bivalve growth, includ- 
ing growth in M. merccnaria (Claereboudt 1994. Mallonee 1989, 
Crockett 1988). Foe and Knight (1985) found that Asiatic clams 
(Corhicuta flwninea) are probably food limited in the Sacramento- 
San Joaquin Delta, even with chlorophyll ti concentrations as high 
as 62.5 p-g/L. which is an order of magnitude higher than concen- 
trations found in Puget Sound. 

Finally, there is also a gradient of human activity at these three 
beaches. Potlatch may be a site of intense harvesting, if not for 
great numbers of butter clams, then certainly for Manila clams. 
Also as noted earlier, there was a commercial harvest by Native 
American harvesters. Birch Bay is not subject to commercial har- 
vest, but is heavily used for recreational purposes, since the road- 
way allows direct road access to the beach. Double Bluffs is also 
not available to commercial harvesting and probably has little 
recreational use of the butter clam resource due to distance of the 
clam bed from the road access. One effect of heavy use may be 
substrate compaction. Studies conducted in Washington demon- 



strated that substrate compaction may be an important cause of iow 
productivity on ciam beaclies (Toba et ai. 1992). Increased liuman 
activity in an area Inas been sliown to decrease biological integrity, 
resulting in decreased species diversity, and may have negative 
impacts on growth and health in general on all taxa in a given 
ecosystem (Karr 1981 ). Houghton ( 1973) showed a high correla- 
tion between taxa richness and growing areas with high clam den- 
sities, strong growth, and high productivity. Thus, bivalve growth 
may be negatively coiTelated with a gradient of human activity. 

Indeed, this may be an important factor in growth on popular 
recreational clam beaches, although it is probably negligible on 
more remote beaches, such as Double Bluffs. 

Patchiness in age distribution may have affected growth analy- 
sis for Potlatch State Park. In the von Bertalanffy analysis for 
Potlatch. the lack of strong representation of older clams likely 
contributed to the computation of an unrealistic L-,_. causing the 
Potlatch curve to eventually cross the others. If the true asymptote 
for Potlatch is closer to those of the other beaches as is suspected, 
the curve would probably not cross the others. It should be noted 
that the calculated parameters are still adequate to describe the 
observed growth for at least the first four years. Due to poor data 
for subsequent years, the model does not provide an adequate 
description of growth for later years. 

CONCLUSIONS 

1 . Butter clams from different beaches in Puget Sound. Wash- 



ington have different rates of growth. Fastest growth oc- 
cuned at Double Bluffs, and slowest growth occurred at 
Potlatch. 

2. The legal size limit for butter clams in Washington probably 
should be increased. The current size limit for butter clams 

is 38 mm. According to results of growth analysis, clams in 
this size class would be only about 2-3 years old. It is 
suggested that the size limit be increased to about 63 mm, 
the size limit in British Columbia. Clams in this size class 
would be about six years old, and would have had oppor- 
tunities to spawn. 

3. Since rates of growth are different in different growing ar- 
eas, it is suggested that a more localized management ap- 
proach for the resource be utilized in order to protect the 
productivity of the stock. A generalized management strat- 
egy will not optimally manage the resource. This would 
require growth analyses to be performed in any area that has 
any commercial or intense recreational harvests for butter 
clams. 

ACKNOWLEDGMENTS 

The senior author wishes to express his appreciation to his 
Supervisory Committee for their invaluable guidance, Drs. Ken- 
neth Chew. Neil Bourne, and Loveday Conquest. Thanks also to 
my field crews for all of their hard work. Finally, special thanks to 



the Graduate School of the University of Washington, the 
Loosanoff Foundation, and the Research, Admissions, and Schol- 
arship Committee of the School of Fisheries at the University of 
Washington for their generous financial support. 

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